Photolithography is a common technique employed in the manufacture of semiconductor devices. Typically, a semiconductor wafer is coated with a layer of light sensitive resist material (photoresist). Using a patterned mask or reticle, the wafer is exposed to projected light from an illumination source, typically actinic light, which manifests a photochemical effect on the photoresist, which is ultimately (typically) chemically etched away, leaving a pattern of photoresist "lines" on the wafer corresponding to the pattern on the mask or reticle. The patterned photoresist on the wafer is also referred to as a mask, and the pattern in the photoresist mask replicates the pattern on the image mask (or reticle).
As used in the main, hereinafter, with respect to semiconductor lithography, the term "upstream" means towards the illumination or radiation source, and "downstream" means away from the illumination source (or, towards the wafer). For example, a lens in the illumination path of photolithographic apparatus has an upstream side facing the illumination source and a downstream side facing away from the illumination source.
FIG. 1 shows a simplified prior-art photolithographic apparatus 110 for exposing a semiconductor wafer (W), more particularly a coating thereon (e.g., photoresist), to light. An optical path is defined from left to right in FIG. 1, as viewed. Prior to exposure, the semiconductor wafer (W) typically receives on its front surface a layer of photoreactive material (not shown), such as photoresist.
A light source 112 emits actinic light, and may be backed up by a reflector 114. Light emitted by the light source typically passes through a uniformizer 116, such as a "fly's eye" lens or a light pipe.
Light exiting the uniformizer 116 is represented by rays 118a, 118b, and 118c, and passes through a condenser lens 120. The ray 118b represents the optical axis of the photolithographic apparatus. The light source 112, reflector 114, uniformizer 116 and condenser lens 120 form what is termed an "illuminator", which is often detachable as a unit from the photolithographic apparatus.
An image mask 122 ("M") is disposed "downstream" of the condenser lens 120, at the focal plane (point) thereof. One type of image mask used in the photolithography process is a chromed glass or quartz plate bearing the pattern to be projected onto the photoresist layer. Light is projected through the image mask, and those areas of the image mask which are not chromed allow the light to expose the photoresist, while those areas of the image mask which are chromed prevent the light from exposing the photoresist. The exposed areas of the photoresist typically resist chemical etching, while the unexposed areas can readily be removed, leaving a pattern of photoresist on the surface of the wafer.
Further downstream along the light path, the rays diverge from the mask 122, and pass through a "taking" (imaging) lens 124. Because of its imaging function, the taking lens 124 must be of relatively high quality as compared with the condenser lens 120. The mask 122 is disposed at a common focal point of the two lenses 120 and 124.
A semiconductor wafer (W) is disposed at the "downstream" focal plane, or image plane, of the taking lens 124. Those areas of the mask (or reticle) which are not chromed allow the light to expose a photoreactive layer (e.g., photoresist) on the surface of the wafer (W), while those areas of the mask which are chromed (or otherwise opaquely patterned) prevent the light from exposing the photo-reactive layer. The photoreactive layer is typically a photoresist material. The exposed areas of the photoresist resist chemical etching and, in subsequent processing, are used to form defined features on the wafer (such as on a layer of polysilicon underling the photoresist).
The resist materials used in photolithography are typically organic. Typical resist materials for visible light photolithography include mixtures of a casting solvent, such as ethyl lactate, and novolac resin (diazoquinone).
Inasmuch as the light passing through the image mask (reticle) has an inherent characteristic that induces photochemical activity in the photoresist material, such radiation (e.g., light) is termed "actinic".
In current photolithographic apparatus, light having at least a substantial visible content is typically employed. Visible light has a frequency on the order of 10.sup.15 Hz (Hertz), and a wavelength on the order of 10.sup.-6 -10.sup.-7 meters. The following terms are well established: 1 .mu.m (micrometer) is 10.sup.-6 meters; 1 nm (nanometer) is 10.sup.-9 meters; and 1 .ANG. (Angstrom) is 10.sup.-10 meters.
Among the problems encountered in photolithography are non-uniformity of source illumination and point-to-point reflectivity variations of photoresist films. Both of these features of current photolithography impose undesirable constraints on further miniaturization of integrated circuits. Small and uniformly sized features are, quite evidently, the object of prolonged endeavor in the field of integrated circuit design. Generally, smaller is faster, and the smaller the features that can be reliably fabricated, the more complex the integrated circuit can be.
With regard to uniformity of source illumination, attention is directed to commonly-owned U.S. Pat. No. 5,055,871, issued to Pasch. As noted in that patent, non-uniformities in the illuminating source will result in non-uniformities of critical dimensions (cd) of features (e.g., lines) formed on the semiconductor device, and the illumination uniformity of photolithographic apparatus will often set a limit to how small a feature can be formed. There usually being a small "error budget" associated with any integrated circuit design, even small variations in illumination intensity can be anathema to the design goals.
With regard to reflectivity of photoresist films, it has been observed that minor thickness variations in a photoresist film will cause pronounced local variations in how efficiently the illuminating light is absorbed (actinically) by the photoresist film, which consequently can adversely affect the uniformity of critical dimensions (cd) of features (such as polysilicon lines or gates) sought to be formed in a layer underlying the photoresist. This problem is addressed in commonly-owned, copending U.S. patent application No. 07/906,902, filed Jun. 29, 1992 by Michael D. Rostoker, which discussed techniques for applying a substantially uniform thickness layer of photoresist, and which is incorporated by reference herein.
Another, more serious problem with photolithography is one of its inherent resolution. The cd's of the smallest features of today's densest integrated circuits are already at sub-micron level (a "micron" or ".mu.m" is one millionth of a meter). Such features are only slightly larger than a single wavelength of visible light, severely pushing the limits of the ability of visible light techniques to resolve those features. As integrated circuit features become smaller, the demand for more nearly "perfect" optical components increases. At some point, however, such optics become impractical and inordinately expensive, or even impossible to produce. Although the resolving power of light, vis-a-vis submicron semiconductor features, is being stretched to its limit, the ability to etch (wet, dry, chemical, plasma) features on a semiconductor wafer is not limited by wavelength.
As is well known, ultraviolet light (UV) is slightly higher (in frequency) on the electromagnetic spectrum than visible light. Typically, ultraviolet light has a frequency on the order of 10.sup.15 -10.sup.17 Hz, and has a wavelength on the order of 10.sup.-7 -10.sup.-8 meters. Ultraviolet light is known to be actinic, for example with respect to skin pigmentation. Due to its shorter (than visible light) wavelength, ultraviolet light would seem to hold promise for increased resolution in integrated circuit photolithography. However, it is difficult to find reliable, fluent sources of UV (typically vacuum UV) light. Further, the performance of present day optics begins to degrade substantially at around 190 nm (1.9.times.10.sup.-7 meters; which is towards the top of the visible light spectrum), and is not well suited for focusing UV light.
In contrast to visible light, X-rays have a much shorter wavelength. Typically, X-rays have a frequency on the order of 10.sup.17 -10.sup.20 Hz, and have a wavelength on the order of 10.sup.-8 -10.sup.-11 meters. Evidently, due to their shorter wavelength, X-rays have the inherent capability of providing better resolution than visible light. However, as with UV sources, there are some problems with obtaining reliable emission sources that exhibit good fluence. The best (most intense) X-ray sources (e.g., X-ray tubes) produce X-rays in the range of 1-10 .ANG. in wavelength, with a nominal output spectrum between 2 .ANG. and 6 .ANG. in wavelength.
Gamma-rays exhibit an even shorter wavelength than X-rays. Typically, Gamma-rays have a frequency on the order of 10.sup.19 -10.sup.22 Hz, and have a wavelength on the order of 10.sup.-10 -10.sup.-12 meters. Evidently, Gamma-rays provide the potential for even better resolution than X-rays. Furthermore, gamma-ray sources providing intense streams of fluent emission are readily available, such as in the form of Cobalt-60.
In the absence of the novel viable gamma-ray and X-ray semiconductor-processing techniques disclosed herein, various techniques for "stretching" the resolution of UV and visible light techniques have been contemplated. One such technique provides a method of forming short-channel polysilicon gates (0.6 .mu.m polysilicon feature size). (See, for example, U.S. Pat. No. 5,139,904, issued Aug. 18, 1992 to Auda et al.) This method employs a technique of laying down a layer of conventional photoresist over a polysilicon layer and patterning the photoresist to "normal" dimensions (greater than the ultimately desired 0.6 .mu.m dimension). The photo-resist pattern is then uniformly eroded in all dimensions using an isotropic (non-directional) RIE (reactive ion etching) etch process. The size of features in the photo-resist pattern is carefully monitored during the etch process. When the pattern features are eroded to the desired size, the etch process is stopped. An anisotropic (highly directional) etch process is used to etch away portions of the underlying polysilicon outside of the "shadow" of the eroded photo-resist pattern (relative to a generally vertical etch direction).
While this technique may be employed to produce small polysilicon structures, it has the same limitations as conventional photolithography with respect to line-to-line spacing. Because the photoresist is initially patterned to "conventional" dimensions, it is not possible with such "stretched" techniques to space pattern features substantially closer with sufficient resolution than is ordinarily possible with conventional photolithography.